The radioactive ion beams (RIB) that will be delivered by the SPIRAL cyclotron of the large-scale facility GANIL need a high transmission transport system from the ion source to the target and an efficient detection system. High angular acceptance can and has been achieved using high field superconducting solenoids, such as the SISSI device at Ganil. Here supplementary conditions must be satisfied, especially space around the target must be available for efficient coincidence experiments, and good particle identification must be possible. Choice and optimisation of all constituents for these combined conditions was the subject of the present program.
Hence, the following spectrometer configurations were examined in detail: a gas-filled spectrometer, a big bore superconducting solenoid combined with a dipole analyser, and a superconducting solenoid doublet in front of the existing spectrometer LISE, a velocity filter such as the FMA at Atlas(Argonne, USA). All these solutions included high field superconducting magnets near the target. This however implies very high stray fields, of the order of several tesla near the solenoids. These stray fields render the use of ancillary detectors, in particular of phototubes for detection very difficult. This is why in the final solution a doublet of very huge Q-poles replaces the solenoids. The geometrical aberrations were the object of a difficult and lengthy study. It was found that standard descriptions of fringe fields give completely erroneous results in the case of large solid angles as in the present case. Higher order corrections were found to be necessary, and will be introduced in the final Q-pole construction. A Wien velocity filter was added with respect to the initial project. The final original solution elaborated has the possibility to work in different optical modes.
It renders possible the combination of the characteristics of different spectrometers used in this domain, as resumed in the table below:
Here Soleno, LiseIII and Ship are spectrometers in operation at IPN/Orsay, Ganil/Caen and GSI/Darmstadt respectively. Susan was a project in the U.K. These unique characteristics made us call this spectrometer VAMOS, VAriable MOde high acceptance Spectrometer for identifying products of reactions induced by SPIRAL beams. The flexibility of use is increased further by a variable distance between the target and doublet of Q-poles. Besides the variable optics, the spectrometer is unique by its very big solid angle of 100msr. A big effort was done to define the detection system associated with the spectrometer. The use of light radioactive beams will very often imply the necessity of the detection and identification of very low energy recoil reaction products. Special detectors for such low energies are presently under development. Combination of different detection systems will make it possible to detect and identify ions with energies from 100keV/nucleon to 25MeV/nucleon.
In June 1998, a memorandum of Understanding was signed for the construction of the spectrometer that started immediately after. The spectrometer became operational in mid - 2001. An up to date description can be found on the internet at the address http://www.ganil.fr/vamos/index.html.
This spectrometer will be a reference in the domain of high efficiency spectrometers for secondary beam facilities, a world wide very active domain of research in nuclear physics.
Funding SchemeCSC - Cost-sharing contracts
WA4 4AD Warrington
L69 7ZE Liverpool